1. Field of the Invention
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2008-170041, filed on Jun. 30, 2008, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a communication system for transmitting single-carrier signals by radio and, more particularly, to a radio communication method and device having a function of eliminating interference.
2. Description of the Related Art
For the uplink of next-generation mobile communication, a single-carrier (SC) transmission scheme, which provides a low peak to average power ratio (PAPR), is employed as a radio access scheme because high efficiency of transmission power needs to be accomplished on the terminal side.
Moreover, in wireless packet communication, adaptive modulation and channel coding (AMC) is employed, in which a modulation method is selected depending on the received SINR (Signal to Interference and Noise power Ratio). Specifically, a high-order modulation scheme such as 16 quadrature amplitude modulation (QAM) with high transmission rate is selected when a received SINR is good, and a low-order modulation scheme such as quadrature phase shift keying (QPSK) having relatively high tolerance for noise is selected when a received SINR deteriorates.
However, multipath interference is problematic when data transmission is performed using a single-carrier signal. Deterioration of the reception characteristics is particularly conspicuous when a high-order modulation scheme is used. Various technologies for suppressing this multipath interference are proposed. For example, in Falconer, D., Ariyavisitakul, S. L., Benyamin-seeyar, A., and Eidson, B., “Frequency Domain Equalization for Single-Carrier Broadband Wireless Systems,” IEEE Communications Magazine, Vol. 40, No. 4 (April 2002), pp. 58-66, a frequency domain equalizer is discussed that equalizes multipath interference in the frequency domain at a receiving device receiving a single-carrier signal. In Nakajima, A., and Adachi, F., “Iterative Frequency-domain Interference Cancellation of Inter-code Interference and Inter-antenna Interference,” Technical Report of IEICE (Institute of Electronics, Information and Communication Engineers), Vol. 106, No. 555 (March 2007), pp. 61-64, a receiving device is discussed that generates a multipath interference replica by using a signal after frequency-domain equalization and eliminates a residual multipath interference component in the frequency domain. Moreover, in Futatsugi, Y., and Yoshida, S., “Iterative Frequency Domain Equalization for MIMO Multiplexing in Uplink Single-Carrier FDMA,” in the Proceedings of IEICE Society Conference 2007, B-5-55, a receiving device is discussed that generates a multipath interference replica from bit replicas after decoding.
Moreover, for a technology of eliminating intersymbol interference, for example, a communication system is proposed in Japanese Patent Application Unexamined Publication No. H11-122312 in which the band to be used by a transmission signal is restricted by using a band filter narrower than the Nyquist bandwidth and in which intersymbol interference due to the restriction on the used bandwidth is separated on the receiving side by using a time domain adaptive equalizer.
Further, according to a frequency-time domain hybrid equalizer proposed in Japanese Patent Application Unexamined Publication 2004-530365, a frequency domain equalizer and a time domain equalizer eliminate intersymbol interference by using an error vector generated from a feedback signal before trellis decoding.
As described above, when a high-order modulation scheme is used, the number of signal points is larger than when a low-order modulation scheme is used, and the distance between signal points is small relative to the average symbol power, resulting in the degraded reception characteristics. Particularly in a case of QAM transmission of a single-carrier signal over multipath propagation, residual multipath interference cannot be completely eliminated, resulting in the reception characteristics of QAM modulation greatly deteriorating due to this residual multipath interference.
In addition, the system disclosed in Japanese Patent Application Unexamined Publication No. H11-122312 has the problem that, since intersymbol interference is separated by using the time domain adaptive equalizer, the adaptive equalizer has an increased number of states and therefore the amount of operation increases when the roll-off factor is small and the length of an interfering symbol is large.
According to the equalizer disclosed in Japanese Patent Application Unexamined Publication No. 2004-530365, the accuracy of the error vector generated from a feedback signal before decoding is degraded when large intersymbol interference occurs. Therefore, there is a problem that intersymbol interference cannot be sufficiently eliminated by the frequency domain equalizer and time domain canceller that perform the elimination depending on the error vector.